CreateBundleFromRefList: support the revspecs used for incremental backups

One of the goals of gitaly-backup is to allow for incremental backups. It relies on CreateBundleFromRefList

This is probably implemented by actually sending range revspecs to CreateBundleFromRefList, although that is not so obvious in the source code. In any case, any possible value of ref used by gitaly-backup has to be implemented.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information